Details are given of the construction of an electromagnet, cloud chamber and accessory apparatus for use in cosmic‐ray studies. The magnet has 9100 kg of steel, 1000 kg of copper, and has pole faces 30.5 cm diameter, normally used at an air‐gap of 19.2 cm. The magnet is oil cooled and is usually operated at from 35 to 125 kw, the power being supplied by a motor‐generator with variable field. Under these conditions the field increases from about 12,400 oersteds at the lower power to 16,000 oersteds at the higher power. The cloud chamber is of the diaphragm type, 30 cm diameter by 4.2 cm deep, and is filled with argon and ethyl alcohol vapor to 88 cm Hg pressure. The photographs are taken with the help of a 45° mirror placed against the front glass of the chamber. For illumination a Sperry carbon arc or a capillary mercury arc is used, the latter proving on the whole more satisfactory. With the lower magnetizing currents the temperature of the magnet and chamber can be maintained so close to that of the outside air that convection currents are small, and the tracks show little distortion. Errors in measuring the energy of cosmic‐ray tracks are caused chiefly by distortions of the tracks. The probable error is 6 percent for a track of 109ev, and about equal to the energy itself when this is 1.6×1010ev.